What is Diamond?

Diamond is simply the material that you get when you have a large number of carbon atoms bonded together in a certain arrangement. In this article, we'll break that statement down, bit by bit:

A Large Number

You can get a feel for the number of carbon atoms in a diamond of a given size by referring to its atomic number density. Written in scientific notation, the atomic number density of diamond (at room temperature) is 1.763×1023 cm-3. This means that if you had a cube of pure diamond that was 1 cm on a side (which would represent a rather large diamond indeed), then it would contain approximately 176,300,000,000,000,000,000,000 carbon atoms. This number is about two or three times larger than the probable number of stars in the entire observable Universe!

Now, a diamond of a more reasonable size might have a volume of about 0.01 cubic centimetres, but this would still contain something like 2,000,000,000,000,000,000,000 atoms of carbon. That's two sextillion atoms (in American English), which is roughly the same number as the number of grains of sand comprising all of the beaches on planet Earth. This number is about seven billion times larger than the number of stars in our galaxy, the Milky Way.

Carbon Atoms

Diamond simply is what it is because it is made of carbon atoms. If it were made of any other type of atom, it would be a completely different material, without the superlative properties that diamond is prized for. Carbon is probably the most remarkable of all of the chemical elements; it is the fourth most common element in the Universe (by mass, after hydrogen, helium, and oxygen) and it is capable of forming more chemical compounds than any other element can. Indeed, it is the basis of all known life on Earth, and is likely to also be the basis of any life existing elsewhere in the Universe.

The arrangement of atoms in the diamond structure. The spheres represent carbon atoms, while the rods signify the chemical bonds between them.

Bonded Together

A single carbon atom can form a chemical bond with up to four other atoms (carbon or otherwise); this is because chemical bonds are formed through the sharing of electrons, and a carbon atom has four electrons available for sharing with other atoms. In diamond, this bonding realises its full potential: except at the very surface of the diamond, every carbon atom is bonded to four neighbouring carbon atoms. Because all of the bonds are “used up” in this way, a diamond, once formed, is not easily contaminated by other atoms of a different type.

A Certain Arrangement

The three-dimensional arrangement that carbon atoms adopt in diamond is named after the material: it is known as the diamond cubic crystal structure. A representation of this arrangement is shown in the figure to the right. In diamond, each sphere would be a carbon atom, and each rod would be a chemical bond between neighbouring carbon atoms. The cube indicated by the black lines shows a repeating unit; in an extended diamond crystal, this box is repeated in all three dimensions.

Summary

While it can be said that diamond is simply a certain arrangement of carbon atoms, both parts of this statement are equally important:

If the arrangement of carbon atoms were any different, the material would not be diamond; it might be graphite, or something like coal or soot, and its properties would likely be very different.

If the atomic arrangement were the same, but the atoms were of a different element, again it would not be diamond. For example, atoms of silicon can adopt exactly the same atomic arrangement as diamond, although the resulting crystalline silicon is a shiny, dark-grey/silvery-blue, opaque solid (from which microchips are made).

So, we see that it is the combination of the special atomic arrangement with the fact that the atoms are those of the element carbon that gives diamond its remarkable and celebrated material properties.